1. Field of the Invention
This invention relates to a photoconductive member having a sensitivity to an electromagnetic wave such as light [herein used in a broad sense, including ultraviolet rays, visible light, infrared rays, X-rays and gamma-rays].
2. Description of the Prior Arts
Photoconductive materials, which constitute photoconductive layers in solid state image pickup devices or in electrophotographic image forming members in the field of image formation, are required to have a high sensitivity, a high SN ratio [Photocurrent (I.sub.p)/Dark current (I.sub.d)], absorption spectral characteristics matching to those of an electromagnetic wave to be irradiated, a rapid response to light, a desired dark resistance value as well as no harm to human bodies during usage. Further, in a solid state image pickup device, it is also required that the residual image should easily be treated within a predetermined time. In particular, in case of an image forming member for electrophotography to be assembled in an electrophotographic device to be used in an office as office apparatus, the aforesaid harmless characteristic is very important.
From the standpoint as mentioned above, amorphous silicon [hereinafter referred to as a-Si] has recently attracted attention as a photoconductive material. For example, German Laid-open Patent Applications Nos. 2746967 and 2855718 disclose applications of a-Si for use in image forming members for electrophotography, and German Laid-open Patent Application No. 2933411 an application of a a-Si for use in a photoelectric transducing reading device.
However, the photoconductive members having photoconductive layers constituted of a-Si of prior art have various problems in electrical, optical and photoconductive characteristics such as dark resistance value, photosensitivity and response to light, environmental characteristics in use such as humidity resistance, and stability, which should further be improved. Thus, in a practical solid state image pickup device, reading device or an image forming member for electrophotography including wide scope of applications, they cannot effectively be used also in view of their productivity and possibility of their bulk production.
For example, when applied in an image forming member for electrophotography, residual potential is frequently observed to remain during use thereof. When such a photoconductive member is repeatedly used for a long time, there will be caused various inconveniences such as accumulation of fatigues by repeated uses or so called ghost phenomenon wherein residual images are formed.
Further, for example, according to a number of experiments conducted by the present inventors, a-Si material constituting the photoconductive layer of an image forming member for electrophotography, while it has a number of advantages, as compared with Se, CdS, ZnO or OPC (organic photoconductive materials) such as PVCz or TNF of prior art, is also found to have several problems to be solved. Namely, when charging treatment is applied for formation of electrostatic images on the photoconductive layer of an image forming member for electrophotography having a photoconductive member constituted of a mono-layer of a-Si which has been endowed with characteristics for use in a solar cell of prior art, dark decay is markedly rapid, whereby it is difficult to apply a conventional photographic method or, in some cases, no charge can be retained at all until development.
In particular, in these points to be improved, it is important in practical application of an a-Si type image forming member for electrophotography to make attempts to lower the dark decay during charging treatment and the residual potential at the light portion when irradiated with an electromagnetic wave.
As an approach to solve the problem in this respect, there may be mentioned provision of a so called barrier layer, having the function to impede injection of free carriers into the photoconductive layer from the side of a support during charging treatment, between the support and the photoconductive layer, as has been applied in Se type or CdS type image forming members for electrophotography of prior art.
Such a proposal to provide a barrier layer between a support and a photoconductive layer may also be effective to some extent in a-Si type image forming members for electrophotography. But, under the present situation, due to the restriction in characteristics for affording simultaneously high sensitivity and high dark resistance of the a-Si type photoconductive layer, the range for optimum design of a-Si type image forming members for electrophotography is narrowed, whereby selection of materials satisfying productivity including cost can be possible only within a very limited range.
For example, when a barrier layer is provided between a support and an a-Si type photoconductive layer according to the method of prior art for the purpose of overcoming the above problem, the capacity to be charged is gradually lowered with repeated formations of electrostatic images until there will occur destruction of electric insulation in the barrier layer, thereby forming so called image defect spots on the transferred toner image. When the layer thickness of the barrier layer is increased in order to prevent such an insulation destruction, then the residual potential at the light portion is increased with increase of the layer thickness. As the result, so called fogging of the transferred toner image is caused, which is also another principal factor to lower the quality of the transferred image.
These points make the degree of freedom in optimum design of a-Si type photoconductive members narrower, thus making more severe selection of materials as well as of optimum designing conditions.
Accordingly, it is requirqd in designing of a photoconductive material to make efforts to obtain desirable characteristics as described above along with the improvement of a-Si materials per se.
In view of the above points, the present invention contemplates the achievement obtained as a result of extensive studies made comprehensively from the standpoints of applicability and utility of a-Si as a photoconductive member for image forming members for electrophotography or solid state image pickup devices. It has now been found that a photoconductive member elaborated to have a specific layer structure comprising a photoconductive layer constituted of a so called hydrogenated amorphous silicon, a halogenated amorphous silicon or a halogen-containing hydrogenated amorphous silicon [hereinafter referred to comprehensively as a-Si(H, X)], which is an amorphous material containing at least one of hydrogen atom(H) and halogen atom(X) in a matrix of silicon, characterized by having a specific barrier layer as specified below interposed between said photoconductive layer and a support for supporting said photoconductive layer, is not only satisfactorily of practical use by overcoming all the problems as mentioned above but also has characteristic superior in substantially all respects to those of the photoconductive members of prior art, especially markedly excellent characteristics as a photoconductive member for electrophotography. The present invention is based on such a finding.